This invention relates to a novel method for ablating a coded marking into a glass workpiece and particularly, although not exclusively, to a method for ablating a machine-readable coded marking into a coating on a glass-envelope part of a CRT (cathode-ray tube) using a laser beam.
U.S. Pat. No. 4,327,283 issued Apr. 27, 1982 to P. M. Heyman et al. describes a glass workpiece that carries a machine-readable coded marking, such as a bar-code marking, in its external surface. In one form, the glass workpiece comprises a main body, a thin, dark-colored undercoating on a portion of the external surface of the body, and a thin, light-colored overcoating on the undercoating. The marking comprises a related sequence of substantially-parellel bars recessed into and through the overcoating. The marking may be made by depositing the coatings, each of which consists essentially of pigment particles and an alkali silicate binder, on a selected area of the workpiece and then recessing the marking into and through the overcoating. Recessing can be achieved by ablating the overcoating with a laser beam.
The coatings disclosed in the Heyman et al. patent, which contain a single-component alkali-silicate binder, are improved upon with coatings containing mixtures of three alkali silicates, as disclosed in the patent application entitled, "Method of Making a Coded Marking in a Glass Workpiece Employing a Trisilicate Coating and Product Thereof," filed concurrently herewith by S. B. Deal et al. Such improved coatings can be produced very rapidly (e.g., in less than 60 seconds) on an automatic machine which will also ablate the marking into the workpiece. Still further improvements are desirable while retaining the improvements realized by the novel method disclosed in the Deal et al. application. It is particularly desirable to improve the resistance of the coatings to the degrading effects of the laser beam during the ablation step.